A wireless communication system includes a number of base stations, scattered over a geographic area, to provide service for a number of wireless devices, such as personal data assistants (PDAs), cellular phones, pagers, smart phones and other suitable devices that can move throughout a geographic area. Communication between the base stations in the wireless communication system may be either intercell synchronous or intercell asynchronous. The base stations in an intercell synchronous system are synchronized with each other, whereas the base stations in an intercell asynchronous system are not synchronized with each other. Additionally, the base stations in an intercell asynchronous system do not share a common time reference, and as a result these base stations transmit messages at unsynchronized times relative to each other. Interim Standard IS-95-A is one example of an intercell synchronous system. The wideband code division multiple access (WCDMA) standard is an example of an intercell asynchronous system.
The base stations generally serve as nodes in the wireless communication system, from which links are established between the wireless devices and each base station. Each base station may establish a link with a wireless device by allocating one or more control channels and one or more dedicated channels to the link. A control channel is used for transmitting base station identification and paging information. The dedicated channel may, for example, include a traffic channel to carry voice and data information between the wireless device and the base station. Accordingly, each link represents collectively one or more control channels and one or more dedicated channels to provide communication between the wireless device and each base station.
Synchronization of wireless devices in an asynchronous communication system is commonly achieved by transmitting one or more synchronization signals from each base station to a wireless device. The transmitted synchronization signals provide a timing reference for synchronizing the wireless device's despreading operations. For example, in a WCDMA-compliant system, a common synchronization procedure utilizes synchronization channels (or a code from a common set of synchronization codes, such as a set of downlink pseudonoise (PN) codes) associated with the control channel on a corresponding link. The asynchronous wireless communication system maintains the synchronization channels for each base station in the system. Additionally, each base station maintains a list of neighboring cells and their corresponding synchronization channels for broadcast in each cell.
Soft handovers allow the wireless device to establish links with multiple base stations simultaneously. In order to facilitate a soft handover, the wireless network directs each base station to adjust its transmit signal to the wireless device in an aligned manner such that the wireless device may simultaneously communicate with each base station. As the wireless device travels through the wireless communication system, the wireless device constantly scans for base stations in order to identify potential handover candidates. For example, during a call, or during any other suitable time such as during a registration procedure, as known in the art, the wireless device maintains an active link set list corresponding to the links in use during the soft handover. The wireless device provides measurements, such as signal strength, quality measurements and timing information to the base station for each of the links in the active link set list. The base station then evaluates the measurements and provides an active link set update message to the wireless device to add or delete links during the soft handover.
Asynchronous systems facilitate a soft handover by synchronizing communication between the links established with the wireless device and each corresponding base station. Each base station transmits respective control channel synchronization information over a corresponding link at unsynchronized times relative to each other. One link is selected as a reference link such that the other links are time adjusted to be aligned with the reference link. The wireless device in a soft handover calculates and reports to the wireless communication system the time difference between a current reference link corresponding to a current reference base station, and a new link corresponding to the new base station. The wireless communication system notifies the new base station of the time difference between the links. The new base station can then adjust its receive and transmit timing according to the signal intended for the wireless device in order to compensate for the time difference between the current reference link associated with the current base station and the new link associated with the new base station.
During the soft handover procedure, the wireless device adds and deletes links in order to provide optimum communication between the wireless device and the wireless communication system. For example, the wireless device may add or delete links due to a time-varying signal quality, due to a movement of a user, changes in interference levels, or changes in the propagation environment. Adding and deleting links may be performed by periodically transmitting an active link set update message to modify an active link set list. As is known in the art, newly established links are determined for each frame based on the links from the previous frame and any additions or deletions relative to the current frame.
According to one method, when the wireless device receives the active link set update message, the wireless device adds and deletes links in the order in which the links are provided in the active link set update message, as long as the active set is not full. If the active link set update message requires the wireless device to delete the current reference link, the wireless device will select an alternative link in the active link set list. If, however, the reference link is also marked for deletion in the active link set update message, then the wireless device will have needlessly selected a reference link just to have it removed in response to the active link set update message. Accordingly, the wireless device will then have to reselect another reference link.